Generally, in a construction machine such as a wheel loader and an agricultural machine such as a tractor, an HST drive circuit can be used as a power transmission mechanism to a travel device, and therein, in some cases, a hydraulic pump (HST pump) and a hydraulic motor (HST motor) are stored in one case, and in some cases, these are separately stored in a pump case and a motor case.
Meanwhile, in the HST drive circuit, because hydraulic fluid circulates in the closed circuit between the HST pump and HST motor, it is desirable to provide a cooling function to suppress a rise in temperature of the circulating hydraulic fluid. Therefore, a conventional structure that an oil cooler is arranged at an upstream side of a charge circuit (replenishing circuit) for replenishing an HST closed circuit with fluid (see Japanese Published Unexamined Patent Application No. H6-3720, for example) has been known.
However, in the HST drive circuit, normally, pressure fluid of the charge circuit is to be introduced into variable displacement units of the HST pump and HST motor so as to operate the variable displacement units. Accordingly, as in the above-described patent document, if an oil cooler is arranged at an upstream side of the charge circuit, in, for example, a case such as starting in winter, there is a concern that the fluid temperature of the charge circuit is excessively lowered to heighten oil viscosity, thus starting the variable displacement units becomes difficult. Furthermore, as described above, in an HST drive circuit wherein the HST pump and HST motor have been separately stored in a pump case and a motor case, it is necessary to sufficiently cool both, therein exists a problem to be solved by the present invention.